The present invention relates to a fuel evaporator for promoting the vaporization of the fuel contained in the intake mixture of an internal combustion engine of a vehicle such as an automobile.
Ceramic material such as barium titanate (hereinafter will be called PTC ceramic) has such characteristics that when electrified, it generates heat and when the temperature thereof reaches a specific value called the Curie temperature, the electric resistance thereof increases rapidly.
Under the Curie temperature, the PTC ceramic receives a large amount of electric current to reach the Curie temperature rapidly.
Over the Curie temperature, the electric resistance becomes remarkably large and the PTC ceramic receives only a small amount of electric current. Therefore, the PTC ceramic is not overheated. Thus, the PTC ceramic adjusts the temperature thereof by itself into about the Curie temperature, that is about 120.degree. C. to 150.degree. C.
Since this temperature range is suitable for promoting the vaporization of the fuel contained within the intake mixture of the internal combustion engine, the PTC ceramic has been employed as a heating means of a heater portion disposed directly under a throttle valve within an intake pipe.
In the conventional fuel evaporator using the PTC ceramic as a heating element, the PTC ceramic is accomodated within a casing made of a heat conductive metallic plate and the casing is disposed in a hole formed in the wall of the intake pipe directly under the throttle valve.
In this case, one portion of the casing is exposed to the interior of the intake pipe to form a heating plate portion. To the reverse surface of the heating plate portion, a heating element is closely fixed. A cylindrical supporting portion of the casing, which is continued from the heating plate portion, is closely inserted within the hole of the intake pipe and fixed thereto.
The heating plate portion of the casing is required to instantaneously evaporate the fuel droplets which collide against the heating plate portion. In order to satisfy the above requirement, the temperature of the heating plate portion must be kept over 80.degree. C., preferably 100.degree. C. to 120.degree. C.
The above described PTC ceramic itself has such a characteristics as to satisfy this temperature condition. But from the heating plate portion to which heat is transmitted from the heating element, heat is taken away by the fuel droplets colliding thereagainst. Furthermore, heat escapes from the supporting portion of the casing to the intake pipe which is contacted with the casing. Also, since the temperature of the coolant for the engine which is introduced in the wall of the intake pipe in order to heat the intake mixture is low at an engine starting time, the heat of the casing is taken away by the coolant through the intake pipe.
Therefore, the temperature of the heating plate portion does not rise as desired.
In order to make the temperature of the heating plate portion rise rapidly and to keep the temperature thereof at a temperature above 80.degree. C., it was proposed to form the heating plate portion of a metallic plate and to form the supporting portion of an adiabatic material such as a synthetic resin, ceramic, etc. However, the strength of synthetic resin and ceramic is generally smaller than that of metal. When a strong attracting force is applied to the heating plate portion due to the intake pipe negative pressure, the synthetic resin and ceramic of the supporting portion which is fixed to the intake pipe are in danger of being broken.
Furthermore, when the supporting portion is formed of ceramic or synthetic resin, it is impossible to ground the electric current to the intake pipe through the supporting portion and then a complex ground system is required.
Accordingly, an object of the present invention is to provide a fuel evaporator wherein the heat of the heating element made of PTC ceramic is effectively transmitted to the heating plate portion with hardly any heat escaping from the heating plate portion to the intake pipe.
Another object of the present invention is to provide a fuel evaporator which is not broken when subjected to the intake pipe negative pressure or the vibrations and has excellent durability.
Still another object of the present invention is to provide a fuel evaporator having a simple electrifying structure of the heating element made of PTC ceramic.